In the manufacture of paper an aqueous cellulosic suspension or slurry is formed into a paper sheet The cellulosic slurry is generally diluted to a consistency (percent dry weight of solids in the slurry) of less than 1 percent, and often below 0.5 percent ahead of the paper machine, while the finished sheet must have less the 6 weight percent water. Hence the dewatering aspects of papermaking are extremely important to the efficiency and cost of the manufacture.
The dewatering method of the least cost in the process is drainage, and thereafter more expensive methods are used, for instance vacuum, pressing, felt blanket blotting and pressing, evaporation and the like, and in practice a combination of such methods are employed to dewater, or dry, the sheet to the desired water content. Since drainage is both the first dewatering method employed and the least expensive, improvement in the efficiency of drainage will decrease the amount of water required to be removed by other methods and hence improve the overall efficiency of dewatering and reduce the cost thereof.
Another aspect of papermaking that is extremely important to the efficiency and cost of the manufacture is retention of furnish components on and within the fiber mat being formed during papermaking. A papermaking furnish contains generally particles that range in size from about the 2 to 3 millimeter size of cellulosic fibers, to fillers at a few microns, and to colloids. Within this range are cellulosic fines, mineral fillers (employed to increase opacity, brightness and other paper characteristics) and other small particles that generally, without the inclusion of one or more retention aids, would in significant portion pass through the spaces (pores) between the cellulosic fibers in the fiber mat being formed during papermaking.
One method of improving the retention of cellulosic fines, mineral fillers and other furnish components on the fiber mat is the use of a coagulant/flocculant retention system, added ahead of the paper machine. In such a system there is first added a coagulant, for instance an inorganic coagulant such as alum (aluminum sulfate), or a cationic starch, or a low molecular weight synthetic cationic polymer to the furnish. Such a coagulant generally reduces the negative surface charges present on the particles in the furnish, particularly the surface charges of the cellulosic fines and the mineral fillers, and thereby accomplishes some degree of agglomeration of such particles. After the addition of such coagulant, and after the various significant shear steps of the refining process, there is then added a flocculant. A flocculant generally acts by bridging between particles. A flocculant such as a synthetic anionic polymer is generally fixed onto the furnish particles through the previously added coagulant material which, having been to some extent adsorbed onto the anionic surfaces within the furnish, provides sites of attachment for the anionic flocculant. The synthetic anionic flocculants generally have a thin, flexible nature, and hence are added at a point providing sufficient time lapse before sheet formation to permit the polymer to reach the attachment surfaces, but not so long as to allow polymer reconfiguration. For similar reasons, such retention systems are deemed shear sensitive, and significant shear conditions are to be avoided at least after the flocculant addition.
As noted above, the flocculant of such a coagulant/flocculant retention system bridges the particles and/or agglomerates already formed by the coagulant, from one surface to another, binding the particles into large agglomerates. The presence of such large agglomerates in the furnish as the fiber mat of paper sheet is being formed increases retention. The agglomerates are filtered out of the water onto the fiber web, where unagglomerated particles would, to a great extent, pass through such paper web.
A flocculated agglomerate generally does not interfere with the drainage of the fiber mat to the extent that would occur if the furnish were gelled or contained an amount of gelatinous material. Nonetheless when such flocs are filtered by the fiber web the pores of the web are generally reduced to a degree, reducing drainage efficiency therefrom. Thus the increased retention provided by a retention system may be achieved with a concomitant lessening of drainage efficiency.
Another type of retention system is described in U.S. Pat. Nos. 4,753,710 and 4,913,775, inventors Langley et al., issued respectively Jun. 28, 1988, and Apr. 3, 1990. In brief, such method adds to an aqueous cellulosic papermaking suspension first a high molecular weight linear cationic polymer, followed by subjecting the suspension to high shear conditions, and then adds bentonite prior to sheet formation.
A further type of retention system is described in "Microparticles in Wet End Chemistry", Kurt Moberg, Retention and Drainage Short Course, 1989, Washington, D.C., TAPPI Press, Altanta, Ga. In brief, such "microparticle" system starts with the addition of cationic starch, followed by the additional of colloidal silica.
Greater retention of fines and fillers permits, for a given grade of paper, a reduction in the cellulosic fiber content of such paper. As pulps of less quality are employed to reduce papermaking costs, the retention aspect of papermaking becomes even more important because the fines content of such lower quality pulps is greater generally than that of higher quality pulps.
Greater retention of fines, fillers and other slurry components reduces the amount of such substances that are lost to the white water, and hence reduces the amount of material wastes, the cost of waste disposal, and the adverse environmental effects therefrom.
Another important characteristic of a given papermaking process is the formation of the paper sheet produced. Formation is determined by the variance in the light transmission within a paper sheet, and a high variance is indicative of poor formation. As retention increases to a high level, for instance a retention level of 80 or 90 percent, the formation parameter generally abruptly declines from good formation to poor formation, It has been at least theoretically postulated that as the retention mechanisms of a given papermaking process shift from floc filtration to floc adsorption, the deleterious effect on formation, at high retention levels, will diminish. A good combination of retention and formation is attributed to the use of bentonite in U.S. Pat. No. 4,913,775, noted above. Improved dewatering and a larger fraction of retention by adsorption rather than filtration, is attributed to the cationic starch/colloidal silica system in "Microparticles in Wet End Chemistry" noted above.
It is generally desirable to reduce the amount of material employed for given purposes in a papermaking process, if such reduction can be achieved without significantly diminishing the result sought. Such add-on reductions may realize both a material cost savings and handling and processing benefits. The reduction in concentration of an add-on employed may in instances advantageously diminish various deleterious effects of such add-on. For instance, high levels of alum may result in deposit problems on the machine, and be detrimental to dry strength properties.
It is also advantageous to employ additives that can be delivered to the paper machine without undue problems, if such additives are available for the given purpose. Additives that are easily dissolved or dispersed in water reduce the energy and expense of delivering them to the paper machine and provide a more reliable uniformity of feed.